Nowadays, optical discs as represented by DVDs (Digital Versatile Discs) or BDs (Blu-ray Discs) are widely used as computer-readable recording media and video recording media, utilizing large capacity, random accessibility, portability, and storability, which are some of the features of the optical discs. In recent years, however, as an increase in information quantity accompanied by spread of the Internet, and an increase in information quantity accompanied by high vision TV broadcasting or personal video services have been carried out, there is a demand for a storage suitable for supplying or storing the contents of information. It is necessary to provide higher-density and larger-capacity optical discs than BDs, as data recording media.
A small light collecting spot is formed on an optical disc as represented by BD by a lens. Information is recorded on the optical disc, and information recorded on the optical disc is reproduced with use of laser light. The diameter of a light collecting spot is proportional to the wavelength λ of incident light, and is inversely proportional to the numerical aperture (hereinafter, called as NA) of an objective lens. It is necessary to shorten the wavelength of laser light and to increase the NA in order to further increase the recording density of an optical disc. However, the improvement on recording density has already reached a limit due to the diffraction limit of light.
In recent years, there are proposed optical recording media using near-field light in order to achieve high density beyond the diffraction limit of light. Near-field light is the light generated on the surface of a material when the material is irradiated with light, and is the light that exists only in a region in proximity to the material surface without propagation. The near-field light has a size substantially equal to the size of a small-size material. Accordingly, it is possible to utilize the light of a size not larger than the diffraction limit.
Further, in the case where recording marks are formed on a phase-change recording material, with use of a light spot of a size not larger than the diffraction limit, thermal diffusion occurs in the phase-change recording material heated by the light spot. This may make the size of the recording marks larger than the size of near-field light. Forming a pattern in advance on a recording medium makes it possible to stably perform recording and reproduction, while suppressing heat diffusion. It is possible to implement high-density and large-capacity optical recording media by recording smaller recording marks with use of these techniques (e.g. see patent literature 1).
However, the region where near-field light forms a light spot of a size not larger than the wavelength of light is a region away from the material surface by about several ten nm at most. The light intensity of near-field light exponentially attenuates, as the light is distanced from the material surface. In view of the above, in the case where information is recorded on a recording medium with use of near-field light, it is necessary to make the working distance (WD) between a recording head and a recording medium as small as possible to about several ten nm. As a result, a contact of the recording head with the recording medium may damage the recording head and the recording medium. Thus, the above technique may cause a problem relating to portability and reliability, which are some of the features of optical media.
Further, in recent years, left-handed materials have been drawing attention, as a material having unique optical features. Research and development on the left-handed materials have progressed. Application of the left-handed materials in a variety of fields has been expected.
The left-handed materials have artificial electromagnetic response characteristics such that at least one of a permittivity ∈ and a permeability μ thereof has a negative value. A response between an electromagnetic wave including light and a material is described by way of the permittivity ∈ and the permeability μ. Both of the permittivity ∈ and the permeability μ of a transparent material existing in the nature have positive values, and the refractive index n=(∈μ)1/2 thereof is a real number.
In the case where the permittivity ∈ and the permeability μ of a material simultaneously have negative values, an electromagnetic wave propagates through the material, because the refractive index n of the material is a real number. The materials, in which the relationship between the electric field, the magnetic field, and the wave vector of the electromagnetic wave is a left-handed relationship, are called as left-handed materials (LHMs) or left-handed metamaterials, or simply called as metamaterials. On the other hand, the materials, in which the relationship between the electric field, the magnetic field, and the wave vector of the electromagnetic wave is a right-handed relationship, are called as right-handed materials.
It is reported that the left-handed materials exhibit novel electromagnetic phenomena such as a negative refractive index and a near-field light propagation effect. It is expected that a novel optical element capable of collecting near-field light can be implemented with use of the left-handed materials (see e.g. non-patent literature 1).
Such a left-handed material has microstructures configured such that fragments of a material each having a size smaller than the wavelength of an electromagnetic wave (light wave) to be used are periodically or non-periodically arranged as unit elements. It is impossible to resolute the unit elements by the electromagnetic wave of a wavelength sufficiently larger than the average gap between the unit elements. Accordingly, the left-handed materials behave like a homogeneous material. The electrical properties and the magnetic properties of the left-handed materials are determined by the material quality, the shape, or the pattern of unit elements constituting the left-handed materials. Generally, it is desired to set the size of a microstructure equal to or smaller than about one-tenth of the wavelength of light (see e.g. non-patent literature 2 and non-patent literature 3).
One of the left-handed materials is a left-handed material having a fishnet structure (see e.g. non-patent literature 4). The left-handed material having a fishnet structure has a structure such that net-shaped metal parts and dielectric parts are alternately laminated one over the other. The left-handed material having a fishnet structure is such that a negative permittivity is implemented by plasmon resonance of metal parts extending in the direction of electric field of the electromagnetic wave (light wave) to be used, and that a negative permeability is implemented by metal parts opposing each other and extending in the direction of magnetic field. It is reported that the left-handed material having a fishnet structure can implement a negative refractive index in a light wavelength region.
Further, the left-handed materials have an effect of amplifying near-field light. It is reported that a flat plate made of a left-handed material is formed into a lens having a resolution over the diffraction limit defined by the wavelength of light (see e.g. non-patent literature 5). As described above, use of a lens made of a left-handed material makes it possible to collect light on a small spot, without considering the diffraction limit of light.
There have been proposed a recording medium and a recording system using such a left-handed material as a material for a lens (see e.g. patent literature 2). A lens made of a left-handed material is disposed near a light source or above a recording medium. With use of an effect of amplifying a near-field light component by the left-handed material, it is possible to collect near-field light, which is normally formed only in a region in proximity to a light source, at a position sufficiently away from the light source or from a lens made of the left-handed material. In the above configuration, assuming that the refractive index n of the left-handed material is −1.0, a geometrical imaging condition required by a lens made of the left-handed material satisfies a relationship: d=WD, where d denotes the thickness of the lens made of the left-handed material, and WD denotes the distance between the light exit surface of the lens made of the left-handed material and the light incident surface of the recording medium. Accordingly, increasing the thickness d of the lens made of the left-handed material makes it possible to increase the distance WD. The above configuration contributes to improvement of reliability, which is one of the features of optical media.
However, the left-handed material has anisotropy as well as crystal. In the case of crystal, anisotropy is defined with respect to the direction of crystal axis. In the case of a left-handed material, however, the constant of the left-handed material differs also depending on the electromagnetic wave propagation direction. Accordingly, the light collecting characteristics of a small-size light source incorporated with a lens made of a left-handed material are non-uniform depending on the configuration, the surface area, and the light irradiation position of a microstructure which acts on light, and depending on the electromagnetic wave propagation direction.
In view of the above, in the case where information is recorded on a recording layer having substantially periodically arranged recording regions, with use of the effect of propagation of light including near-field light and the light collecting effect by a left-handed material having substantially periodically formed structures, if the arrangement period of the recording regions and the structure period of the left-handed material do not coincide with each other, recording characteristics and reproduction characteristics may vary.